Furan is a chemical useful in furan resins, but more importantly, it serves as an intermediate for the manufacture of tetrahydrofuran and 1,4-butanediol. Processes for indirect oxidation of butadiene to furan are known. In particular, U.S. Pat. No. 4,172,838 discloses a process for the preparation of furan compounds by catalytic oxidation of a diolefin such as butadiene in an aqueous medium having a pH less than 2 and containing iodide ion, a mixture of cuprous and cupric ions, and a solubilizing agent for cuprous ion such as an alkali metal halide. Regeneration of the cupric ion oxidant is achieved by oxidation of cuprous ion with an oxygen-containing gas and may be carried out by mixing an oxygen-containing gas directly with the diene starting material, or in the second stage of a two-stage process.
S. Torii, Catalyst (Japan), 22, 330-341 (1980), reviews indirect electrolytic oxidation in organic syntheses, that is oxidation which takes place in the bulk solution away from the electrodes using a redox reactant which aids electron transfer in a homogeneous system. In such a system an oxidizing electron carrier is employed as an intermediary between the substrate and the electrodes. First, the oxidizing electron carrier oxidizes the substrate, and then the reduced electron carrier is oxidized at the anode to provide oxidation activity again. This cycle is used for repeated oxidation of the substrate. Thus the indirect electrolytic oxidation process involves the oxidation of a substrate by electrolysis, not directly, but by the regeneration and recirculation of the oxidizing electron carrier.
An example of a commercial application of the technology is the ECRU SYSTEM for chromic acid regeneration manufactured by Resource Engineering Company. This system is designed for electrolytic regeneration of chromium oxidizing solutions, e.g., plastic etches, brass and aluminum bright dyes, anodizing solutions and organic oxidants, by generating chromic acid from chromium (III).